In recent years there has been interest in developing shoulder joints for robots. Shoulder joints approximate the motion of a human shoulder. These joints provide two degrees of freedom by rotating about more than one axis of rotation. The use of shoulder joints gives robots more flexibility of movement within a limited work envelope.
Previous attempts at shoulder joints have been hindered because of their complexity or bulkiness. Most attempts at shoulder joints can be described as roll-pitch-roll mechanisms. These joints use complicated mechanisms with many interlocking gears. Roll-pitch-roll joints tend to be very bulky and have many moving parts. They also suffer from singularity problems. There have been many variations of the roll-pitch-roll mechanism but all variations have been bulky and complex.
Another type of shoulder joint which has been attempted in the past is known as the parallel mechanism. Such a mechanism is described in the article entitled "On the Kinematic Design of Spherical Three-Degree-of-Freedom Parallel Manipulators", Clement M. Gosselin and Eric Lavoie, 17th International Symposium on Industrial Robots, Apr. 26, 1987. Parallel mechanisms use two pyramids with one vertex in common which defines the center of the joint. Prismatic actuators join the other corners of the two pyramids and moving the actuators rotates the joint about its center. Parallel mechanisms are limited by having a small work space and limited load carrying capacities.
Other attempts at shoulder joints include several hydraulic servo mechanisms by Mark Rosheim. These are described in U.S. Pat. Nos. 4,194,437 and 4,296,681. These joints suffer because their hydraulic actuation has a slow time constant and can present risk of explosion in certain environments.
Mark Rosheim has also disclosed a spherical joint with tendon actuation in U.S. Pat. No. 4,804,220. This joint also suffers from several problems. The inherent flexibility of the tendons reduces the rigidity of the joint and introduces vibration problems to the joints. Also, because the joint depends on friction between the tendons and the drive pulleys, the joint can only apply a limited force to an end effector.
Other shoulder joints, such as those described in U.S. Pat. No. 4,628,765 by Dien et al and U.S. Pat. No. 4,878,393 by Duta et al, essentially use a universal joint with two perpendicular axes of rotation. These joints are inherently weak and must use large actuators in order to carry a significant load.